StringRef appends itself to std::string efficiently

include/internal/catch_assertionresult.cpp

@@ -65,9 +65,9 @@ namespace Catch {
             expr = m_info.capturedExpression;
         else {
             expr.reserve( m_info.macroName.size() + m_info.capturedExpression.size() + 4 );
-            expr += m_info.macroName.c_str();
+            expr += m_info.macroName;
             expr += "( ";
-            expr += m_info.capturedExpression.c_str();
+            expr += m_info.capturedExpression;
             expr += " )";
         }
         return expr;

include/internal/catch_stringref.cpp

@@ -115,6 +115,11 @@ namespace Catch {
         return os.write(str.m_start, str.m_size);
     }
 
+    auto operator+=( std::string& lhs, StringRef const& rhs ) -> std::string& {
+        lhs.append(rhs.m_start, rhs.m_size);
+        return lhs;
+    }
+
 } // namespace Catch
 
 #if defined(__clang__)

include/internal/catch_stringref.h

@@ -92,6 +92,8 @@ namespace Catch {
 
         auto operator[] ( size_type index ) const noexcept -> char;
 
+        friend auto operator += ( std::string& lhs, StringRef const& rhs ) -> std::string&;
+
     public: // named queries
         auto empty() const noexcept -> bool {
             return m_size == 0;